STUDY ON THE VALENCE STATES AND BEHAVIOR OF Sn IN Pt-Sn/γ-Al_2O_3 CATALYSTS PREPARED BY CO-PRECIPITATION
Li Yong-xi, (Research Institute of Petroleum Processing, Beijing)Zhang Yong-fu (Institute of Shanghai Petrochemical Complex Works)and Shia Yuan-fu (Department of Physics, Nanjing University)
The valence states of Sn in the Pt-Sn/r--Al2O3 reforming catalysts of different Sn contents were studied by Mossbauer spectroscopy and X-ray photoelectron spectroscopy (XPS) in various pretreatment stages. Mossbauer spectra of the catalysts showed the characteristic isomer shift for Sn4+, Sn0 and Sn2+ respectively, but the absorption of Sn4+ in each spectrum of Pt-Sn/r--Al2O3 catalysts prepared by co-precipitation was significantly larger than that of Sn0 and Sn2+. The results were consistent with those obtained from XPS experiments. In addition ,it was found from XPS that the Sn3d5/2 binding energies of catalyst samples were lower than those of standard tin chlorides, and that the C12p binding energies (198.4-198.6eV) of catalyst samples were also lower than those (200.4eV) of SnCl2/r-Al2O3 and SnCl4/r-Al2O3, but very close to that of HC1. Therefore, the chlorine in the catalyst samples seems to be in the form of HC1, which is adsorbed on the support of the catalysts. The Mossbauer parameters were obtained by computer fitting from above results. It was shown that the distributions of tin compounds in the catalysts were .quite the same after reduction, though the contents of tin were different (from 0.3 to 1.5wt%). The Sn in reduced Catalysts presented mainly in the form of Sn4+ (SnAl2O5, SnO2) and partially in Sn2+ (SnAl2O4, SnO) and Sn0.No tin chloride has been found. The alloys Pt-Sn appeared as the result of the action of Sn0 and Pt, the atomic ratios of Sn/Pt in the alloys were changed when tin content in the catalysts increased. The states of tin in catalyst were fairly stable after calcination, reduction and reaction, and remained mainly in Sn4+. No significant changes of the amount of alloy were observed, however, the contents of both SnAl2O5 and SnO2 in Sn4+ species were convertible. From calcination to reduction, the content of SnAl2O5 increased, whereas it decreased after reaction. The amount of SnAl205 after reaction was even less than that after calcination, on the contrary, the amount of SnO2 increased. However, the states of tin in the catalyst prepared by co-precipitation method was much more stable than those by other methods. It is then suggested that there is an interaction between tin oxide and A12O3 support in the catalyst, and this interaction seems to play an important role in reforming reaction as seen from the fact that the catalyst prepared by co-precipitation has excellent catalytic performance.